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Array

Used In

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Reference Manual

VHDL-93:
  • Section 3.2.1

Syntax

type type_name is array (range) of element_type;

Rules and Examples

An array contains multiple elements of the same type. When an array object is declared, an existing array type must be used.

type NIBBLE is array (3 downto 0) of std_ulogic;
type RAM is array (0 to 31) of integer range 0 to 255;
signal A_BUS : NIBBLE;
signal RAM_0 : RAM;

An array type definition can be unconstrained, i.e. of undefined length. String, bit_vector, and std_logic_vector are defined in this way. An object (signal, variable or constant) of an unconstrained array type must have it’s index type range defined when it is declared.

type INT_ARRAY is array (integer range <>) of integer;
variable INT_TABLE: INT_ARRAY(0 to 9);
variable LOC_BUS : std_ulogic_vector(7 downto 0);

Arrays with character elements such as string, bit_vector, and std_logic_vector may be assigned a literal value using double quotes (see literals):

CONSTANT MSG_o: string := "Test 1 Completed";

A_BUS <= "0000";
LOC_BUS <= "10101010";

Arrays may also be assigned using concatenation (&), aggregates, slices, or a mixture. By default, assignment is made by position.

A_BUS <= (A_BIT, B_BIT, C_BIT, D_BIT);

-- an equivalent assignment:
A_BUS <= (A_BIT & B_BIT & C_BIT & D_BIT);

-- rotate A_BUS to the left:
A_BUS <= A_BUS(2 downto 0) & A_BUS(3);

Arrays of arrays may be declared. These are useful for memories, vector tables, etc.:

type MEM is array (0 to 7) of NIBBLE;
-- an array "array of array" type
variable MEM8X4 : MEM;

--

-- accessing the whole array:
MEM8X4 := ("0000", "0001", "0010", "0011", "0100", "0101", "0110", "0111");
-- accessing a "word"
MEM8X4(5) := "0110";
-- accessing a single bit
MEM8X4(6) (0) := '0';

True two (or more) dimensional arrays may also be declared:

type T_2D is array (3 downto 0, 1 downto 0) of integer;
signal X_2D : T_2D;

--

X_2D <= ((0,0), (1,1), (2,2), (3,3));
X_2D(3,1) <= 4;

Synthesis Issues

Most logic synthesis tools accept one-dimensional arrays of other supported types. 1-D arrays of 1-D arrays are often supported. Some tools also allow true 2-D arrays, but not more dimensions.

Note that arrays are usually implemented using gates and flip-flops, not ROM’s and RAM’s.

See Also